1. Field of the Invention
The present invention relates to a method for producing a single crystal silicon solar cell and to a single crystal silicon solar cell, and particularly to a method for producing a single crystal silicon solar cell for forming a single crystal silicon layer on a transparent insulator substrate and to a single crystal silicon solar cell having a single crystal silicon layer over a transparent insulator substrate.
2. Description of the Related Art
Solar cells comprising silicon as main materials are classified into single crystal silicon solar cells, polycrystalline silicon solar cells, and amorphous silicon solar cells, based on crystallinities thereof. Among them, single crystal silicon solar cells are each provided as solar cell elements by cutting a single crystal ingot prepared by a crystal pulling method by a wire saw into a wafer shape slice, working the slice into a wafer having a thickness of 100 to 200 μm, and forming p-n junctions, electrodes, a protective film, and the like thereon.
In case of polycrystalline silicon solar cells, there is fabricated a polycrystalline ingot by crystallizing a molten metal silicon in a mold without relying on crystal pulling, and the ingot is cut into a wafer shape slice by a wire saw in the same manner as single crystal silicon solar cells, and the slice is worked into a wafer having a thickness of 100 to 200 μm, and formed with p-n junctions, electrodes, a protective film, and the like thereon, to provide solar cell elements, in the same manner as a single crystal silicon substrate.
In case of amorphous silicon solar cells, there is formed an amorphous silicon hydride film on a substrate by decomposing a silane gas by discharge in a vapor phase such as by a plasma CVD method, and diborane, phosphine, and the like as doping gases are added thereto, followed by simultaneous deposition thereof to simultaneously achieve a p-n junction formation process and a film-formation process, and followed by formation of electrodes and a protective film, thereby providing solar cell elements. In an amorphous silicon solar cell, since amorphous silicon as a direct transition type absorbs incident light, the amorphous silicon has a light absorption coefficient which is about one order higher than those of single crystal silicon and polycrystalline silicon (“Solar photovoltaic power generation”, p. 233, by Kiyoshi Takahashi, Yoshihiro Hamakawa and Akio Ushirokawa, Morikita Shuppan, 1980), thereby providing an advantage that a thickness of about 1 μm of an amorphous silicon layer will do which is about a hundredth of that of a crystal-based solar cell. Thus, expectation is significant for amorphous silicon solar cells capable of effectively utilizing resources, in view of the fact that the annual production volume of solar cells has recently exceeded 1 giga-watts in the world and the production volume will be further increased.
However, it is inappropriate to determine the effective utilization ratio of resources based on simple comparison with a film thickness required by a crystal-based solar cell, because of exemplary circumstances that high purity gas materials such as silane and disilane are used as starting materials for fabrication of amorphous silicon solar cells, and that the effective utilization ratio of the gas materials includes deposition thereof at locations in a plasma CVD apparatus other than at a substrate. Further, an amorphous silicon solar cell has a conversion efficiency of about 10% whereas a crystal-based solar cell has a conversion efficiency of about 15%, and there is still left a problem of degradation of output characteristic in an amorphous silicon solar cell under light irradiation.
As such, there have been conducted various approaches for developing thin-film solar cells by utilizing silicon crystal-based materials (“Solar photovoltaic power generation”, p. 217, by Kiyoshi Takahashi, Yoshihiro Hamakawa, and Akio Ushirokawa, Morikita Shuppan, 1980). For examples there is deposited a polycrystalline thin-film on an alumina substrate, graphite substrate, or the like, by using a trichlorosilane gas, a tetrachlorosilane gas, or the like. The thus deposited film has a lot of crystal defects, and the conversion efficiency is low as it is. Thus, it is required to conduct zone melting to improve crystallinity, so as to increase the conversion efficiency (see JP-A-2004-342909, for example). However, even by conducting such a zone melting method, there has been still left an exemplary problem that photocurrent response characteristics in a longer wavelength range are lowered because crystal grain boundaries cause a leak current and shorten lifetimes of carriers.